1. Field of the invention
The present invention relates to a restrainable manhole (or handhole) cover and the open/close device therewith, and more particularly to a manhole (or handhole, it may be represented only by manhole thereinafter) cover with a structure which can retain the cover in its position even overlaid or hit by a heavy object as a vehicle, and a cover open/close device which can be used to hang up and remove the manhole cover promptly, conveniently and easily.
2. Description of the Prior Art
In a modern and highly congested city, lots of public facilities, such as electrical power lines, communication wires, and sewer system are installed underground, either in respective underground duct, tunnel, or in a commonly used ductway. Manholes and handholes are therefore required to be built in proper location along the roadway for installation of the necessary equipment, pulling cables, routine inspection and maintenance. As the manhole covers are installed on the surface of the pavement, they are exposed to the danger of being overlaid or hit by heavy vehicles and displaced or even destroyed entailing the severe problems of public security. Some drawbacks of conventional manhole covers experienced in the past are enumerated hereinafter:
1. As being overlaid day and night for a long duration, a manhole cover is gradually worn out and reduces its thickness, and finally cracks. As a replaced new cover and the remaining manhole neck are not formed of the same mold so that the existence of excessive horizontal and vertical clearances between the manhole cover and neck can not be prevented. PA0 2. If the quality of sand used to form the mold for making the manhole cover is poor due to its uniform granular size. The contact surface between the cover and the neck will not be smooth but with an excessive vertical clearance. PA0 3. A manhole cover is deformed due to unappropriate treatment in controlling its temperature during its manufacturing process in sand molding. PA0 4. Imperfect sample test for finished products allows some unqualified products, especially with excessively deviated size, to be considered as qualified. PA0 5. A manhole cover is not properly returned to its original place with some clay or gravels interlaid between the manhole cover and the manhole neck by careless workmanship by installation or maintenance personnel. PA0 6. A particular portion on a manhole cover is overlaid by vehicle wheels in case the manhole is built under the passage of vehicles. That portion of the manhole cover together with that of the manhole neck will wore out severely than other portions. If the manhole cover is not returned exactly to its original position with respect to the manhole neck after opening, cover and neck never keep close matching with each other to make the surface of the manhole cover flat and horizontal with the pavement.
All of the six factors mentioned above will cause a bounce or a displacement of a manhole cover when it is overlaid or hit by a vehicle, or even cracked or whirling away causing danger to public.
A severest highway traffic accident happened on Jan. 23, 1989 was a typical example. A heavy track overlaid a cast iron manhole cover installed on the vehicle passage of Chung Shin Bridge connecting Taipei City with Taipei County. It was a manhole cover for communication duct system. However, the cover bounced up and removed from its position after being hit, which caused a following large passenger bus to stumble over the uncovered manhole and overturned. Unfortunately, the overturning bus collided a car coming from opposite direction just at that moment and caused four persons died who were riding on the car.
The inventor of the present invention investigated the above accident carefully from every angle, the conclusion is described in detail as follows:
According to the design control and criteria concerning Highway Bridge recommended by American Association of State Highway and Transportation Officials (AASHTO), a shaft distance between front and rear wheels for a typical track Dd is 4.25 m, if running speed is 60 km/hr, the diameter of manhole cover RF is 0.676 m, the overlaying begins at the instant, .SIGMA.T dR=0, where dR.gtoreq.0. the time required for the front wheel to pass over from the rear end R to front end F of the manhole cover will be: EQU T.sub.RF =RF/.upsilon.=0.676 m/(60.times.1000 m/hr).times.60.times.60=0.04056 sec.
the time required for the rear wheel of the track to reach the rear end R of the manhole will be EQU TDR=(Dd-RF)/M=(4.25 m-0.676 m)/(60.times.1000 m/hr).times.60.times.60=0.21444 sec.
At the beginning of overlaying, T.sub.dR =0, the manhole cover was supposed to be at its normal position as shown in FIG. 1, and supposing that the manhole cover was not installed exactly vertically, yet tilted slightly with its front end F uprightly aparted from its correct position with a vertical clearance H.sub.1, from top surface of the manhole neck as shown in FIG. 2. After the time T.sub.RF =0.0405 sec. elapsed the front wheel reached point F, the front end of the manhole cover. As the point F was already apart from the manhole neck with a clearance H.sub.1, an instantaneous impact of the wheel made the rear end R of the cover to jump up with a clearance H.sub.2 which being larger than H.sub.1 (H.sub.2 &gt;H.sub.1) . As the front wheel was about to leave the point F, part of the vehicle weight was exerted on the front end of the manhole cover aparting already a vertical clearance H.sub.1 from its normal position. This second impact would further enlarge upright clearance H.sub.2 of the rear end of the cover. The manhole cover displayed a flip flop action as shown in FIG. 3. Finally at the moment T.sub.DR =0.214 sec. when the rear wheel hit the rear end R of the cover, the cover was entirely removed resulting in a severe traffic accident as described above (Referring to FIG. 4).
Further referring to FIGS. 5 through 9, due to a pulling force or compressive force exerted by the vehicle to the manhole cover, the manhole cover might remove from its position by turning counter-clockwise. The suspected state was as follows:
As shown in FIG. 5, doe to a clearance existing between the manhole cover and neck, when the front wheel of the vehicle overlaid one end of the manhole cover, the other end would jump up. As the wheel exerted a pulling force or a compressive force on the manhole cover, the cover obtained a moment of counter clockwise rotation, at this time highest point of jumped up cover also turned in counter clockwise direction as shown in FIG. 6. At the moment the front wheel overlaid rear end of the cover, the highest point of the cover would move right rearward while it was already over the ground as shown in FIG. 7. After the front wheel had passed through, the cover still kept turning upward by its own inertia as shown in FIG. 8 According to the above calculation, the time required for the rear wheel to reach the rear end R of the cover was T.sub.DR =0.214 sec. Apparently in such a short time, the rear wheel would definitely hit the rear end of the flying cover with so huge impact force that threw the cover in right forward direction as shown in FIG. 9.
From the above description, it should be understood that there is still room for improving the design of conventional manhole covers. The inventor of the present invention disclosed a solution to solve the existing problems in conventional products and acquired a patent of R.O.C. by patent No. ZL96207779.8.
Through a consistent effort, the inventor discloses the present invention hereinafter which is more practicable and effective than the former one.